Vacuum tube circuit frequency changing



VACUUM TUBE CIRCUIT FREQUENCY CHANGING Filed July 14, 1932 WITNESSES:INVENTOR Q2 fialob/ll fiarmofi. 0:

A'TTONE Patented Mar. 30, 1937 UNITED STATES OFFICE VACUUM TUBE CIRCUITFREQUENCY CHANGING Pennsylvania Application July 14, 1932, Serial No.622,389

6 Claims.

This invention relates to oscillation generators and particularly tooscillation generators of the vacuum tube type in "which the frequencyis controlled by separate oscillating means.

5 Oscillation generators, particularly for use in connection with radiobroadcasting, must have a very constant frequency. It is usual to obtainthis by producing the frequency from a lower frequency which "is readilycontrolled or is of a 10 very constant character. "The oscillation ofpiezoelectric 'crystals'and of tuning forks are familiar illustrationsof such control frequencies.

In order to obtain a frequency of an order suitable for radiotransmission purposes from a piezo- 15 electric crystal or anelectrically driven tuning fork, it is usual to multiply the frequenciesin a succession of steps in each of which an oscillation generator isused which produces many and strong harmonics, one of which is separatedfrom the 29 others by filtering and impressed upon the input of the nextstep. In order that the output shall contain strong harmonics, it isusual to cause the grid potential to oscillate over an amplitude sogreat that the response thereto in the plate cir- 5 cuit will not belinear. The non-linear character of the response is depended upon forproduction of the harmonics.

In my invention I am able to produce the desired frequency with asmaller number of steps 3 because :Ihave discovered a more effective wayof producing a distorted plate current.

It is an object of my invention to obtain the desired frequency from agiven frequency by means which shall not require many steps and in whichthe adjustments need not be nice.

It is a further object of my invention to provide means for obtaining anoutput of any one of a large number of frequencies from two or morestandard-input frequencies and to hold the output frequency to theselected value with certainty and without-requiring careful adjustment.

It is a further object of my invention to take advantage of acharacteristic of a screen-grid tube, namely that when thescreen-gridpotential is approximately equal to the plate potential the platecurrent becomes very distorted.

It is a further obj ectof my invention to control an oscillationgenerator by means of potentials impressed upon the screen grid.

It is a further objectof my invention to obtain oscillations of a highfrequency by the action of control oscillations of lower frequency withonly a small number of steps.

It is a further object of my invention to provide an oscillationgenerator capable of delivering a large number of different frequenciesand also capable of dependably delivering the selected one of saidfrequencies without requiring any high .degree of care in adjustment toobtain said se I lected frequency.

It is a further object of my'invention to produce. in the output circuita harmonic-equal to a harmonic of a control or of a combination ofcontrol frequencies.

Other objects of my invention and details of the structure employed willbe-evident from the following description and the accompanying drawing,in which the single figure-of drawing is a diagrammatic representationof a circuit embodying the principles of my invention.

In the figure the vacuum tube I is supplied with the usual filament '2,heated by current from a source 3. The plate 4 of the tube l and thecontrol ,grid 5 thereof are connected through the usual circuits toconstitute a Hartley oscillator. The sourceof power for :this oscillatoris the battery 6. A radio-frequency choke .coil 1 between the positiveend of this :battery and the connection to the plate '4 confines thehigh-frequency oscillations to the tuned circuit which comprises aninductor '8 anda condenser :9. The tuning of the circuit is accomplishedby adjusting one of the reactors. In the illustration only the condenser9 is shown :as adjustable but the inductor 8 may also be made adjusable,if desired.

The control grid 5 :is supplied with the usual grid resistor .H and gridcondenser =|2 through which it is connected to the terminal of theinductor 8 opposite that connected to the plate 4. The center point ofthe inductor 8 isconnected to the filament 2 through astoppingzcondenser [-3.

--A stopping condenser 14 is inserted between the filament and aconnection to the screen grid 15. A radio frequency rchoke coil :l-Bprevents the high frequencies impressed uponthe screen grid fromreaching the resistor If! which :is connected across a portion of thebattery "-6 and acts as a potentiometer from which, through vthe radiofrequency choke coil 16, .a direct-current potential is impressed on thescreen .grid. The .connection to the screensgrid includes thesecondaries 20 and 2| of two transformers. The .primaries of thesetransformers :are supplied from any suitable sources with frequencies offixed magnitudes.

In the operation of thedev-ice, the screengrid I5 is at a direct-currentpotential somewhat lower than the potential of the plate 4. The tube,therefore, would, if it were not for the secondaries -til-and 2|,operate as an oscillation generator producing a frequency determined bythe tuning of the circuit 89 and yielding a substantially undistortedplate current.

The potentials introduced by the secondaries 20 and 2| cause the deviceto depart from the ordinary behavior of an oscillator. Each time thatthe potential of the grid 15 is sufficiently increased by the voltagesintroduced by these secondaries 20 and 2| to cause it to be nearly equal10 with the plate potential, the plate current, in-

stead of being a linear function of the potential upon the control grid5, becomes a non-linear I function thereof, that is, the plate current,instead of being of a nearly pure sine-wave form, becomes highlydistorted and contains many harmonies.

As the tuning of the circuit 8-9 is adjusted the frequency of thecurrent therein or some harmonic of that frequency will become equal tothe frequency delivered by the transformer secondary 20, the transformersecondary 2| or the combination of them or some harmonic thereof. IWhenever this occurs, the interaction of the equal harmonicswill besuperposed upon the ordinary feed-back action of the oscillator, withthe result that the particular frequency of current in the circuit 89 atwhich this occurs will be maintained even though the tuning of thiscircuit is materially different from that for which this would be theresident frequency. I

If the adjustment of the circuit 89 be altered continuously from oneextreme value to the other and the current therein observed throughoutsuch alteration, it will be found that the current remains persistentlyof one frequency until the ,v: tuning differs so much from the resonanttuning for said frequency that this is no longer possible and thenchanges to a frequency corresponding to the next value for which theabove-stated re- 40 elation holds. At new frequency, it will againpersist until the tuning has been changed enough to cause it to becomeclose to the tuning for the next frequency value corresponding to saidcondition. In the intervals between the settings of the condensercorresponding to the several frequencies 7 just described the current isunstable. An oscillographic observation of it shows a blur instead .ofindicating any definite frequency. The settings separating suchintervals are not sharp points, for the condenser maybe changed somelittle distance from the adjustment corresponding to exact resonancebefore the current changes from the fixed frequency to the unstablecharacter. The current is stable at any one of a large number offrequencies. Which one, may be determined by the tuning. A gradualchange of frequency cannot be obtained by a gradual change 'of thetuning. Frequencies between those that thus persist cannot be obtainedby any tuning. f The condition that the frequency of the current in thecircuit 8-9 or some harmonic thereof shall equal the frequency impressedby one or the other transformer or by their combined action or someharmonic thereof can be expressed algebraically by the equation (Meme)very large number of ways so that the number of frequencies at which thesystem may be stable is large. If the constants p and q are each unity,the combinationof harmonics of f1 and f2 coact directly with thefundamental of the current in V the circuit 8-9.

If q is greater than unity, while p remains unity, the combinationequals a harmonic of the currentin the circuit 8-9. On the other hand,

' if q.is unity and p is greater than unit, a harmonic of thecombination is coacting with the fundamental in the circuit 89.

As a rule, the higher the index of the harmonic,

the smaller the portion of the total energy present in the current assuch harmonic, consequently it is true in general that the device ismore stable when the quantities n, m, p and q are small. I have foundthat a quite substantial alteration in the setting of the condenser 9may be'made without causing a change in frequency when n and m areneither greater than 10.

The stability is greater when p and q are each unity, particularly ifnand mare in the upper part of the permissible range. Onthe other hand,p may be as large as 10jwithout introducing need for a verycriticalsettingof the condenser. When p is unity,.q may be somewhatlarger than in other circumstancesrbut while' desirable results can beobtained with q as large as 10, it is usually preferable to avoidrequiring a very high harposes from a crystal or a tuningfork using amuch smaller number of tubes than has here tofore been required.

The invention is, however, not limited to this application. It can beused to produce any desirable harmonic or sub-harmonic from onestandardized frequency such as a fork or a crystal frequency. It canalso be used to produce any desired combination from two sources ofconstant frequency.

Although, in the preferred form of myinvention, the controllingfrequencies are introduced into the screen-grid circuit, it is possibleto obtain at least the easier of the results described herein by usingthe control frequency in other places; in the plate circuit, forexample, or in the circuit of the control grid.

Many other variations of the structure will occur to those skilled inthe art. The omission of a specific descriptionlof such variations isnot intended as a limitation. No limitation except those imposed'by theaccompanying claims or required by the prior art is intended.

' I claim as my invention: 7

1. A frequency changer comprising an electron discharge device having atleast four electrodes,

a circuit interconnecting one of said electrodes j with another of saidelectrodes, a second circuit interconnecting the electrode lastmentioned with a different one of said four electrodes, said circuitsbeing coupled and provided with tuning means to. constitute anoscillation generator a 30 monic of the current in the circuit 8-9 forstaplurality of sources of different control frequencies and means forsimultaneously impressing said control frequencies upon the fourthelectrode of said discharge device.

2. A frequency changer comprising an electron discharge device having atleast four electrodes, a circuit interconnecting one of said electrodeswith another of said electrodes, a second circuit interconnecting theelectrode last mentioned with a different one of said four electrodes,said circuits being coupled and provided with tuning means to constitutean oscillation generator, a plurality of sources of control frequenciesand means for simultaneously impressing said control frequencies uponthe fourth electrode of said discharge device, said tuning means beingtuned to oscillate at a frequency which bears the following relationshipto the control frequencies P 3 6 ii z) where 1'1 and f2 are controlfrequencies, and the coefficients P Q n and m are integers.

3. A frequency changer comprising an electron discharge device of thescreen grid type, a circuit connecting one of the grids of said deviceand the cathode, another circuit connecting the anode of said devicewith said cathode and including a source of potential, said circuitbeing coupled to constitute said discharge device, an oscillationgenerator, a control frequency potential and means for impressing saidpotential upon the remaining grid of said discharge device, means tomaintain said grid normally below the potential of said anode, thecontrol frequency potential being of such value as to raise thepotential of said remaining grid to the value of said plate potential atleast once in each period.

4. In combination with an electrical discharge device having a cathode,an anode and at least two control electrodes, a source of energy and aconnecting circuit between said anode and cathode, a coupling betweensaid circuit and one of said control electrodes adapted to cause saiddischarge device to produce spontaneous electrical oscillations, aresonant means connected in said circuit to determine the frequency ofsaid oscillations, a source of periodic energy in circuit with the otherof said two control electrodes, the resonant frequency of saidoscillations being substantially a multiple of the frequency of saidlast-named source.

5. In combination with an electrical discharge device having a cathode,an anode and at least two control electrodes, a source of energy and aconnecting circuit between said anode and cathode, a coupling betweensaid circuit and one of said control electrodes adapted to cause saiddischarge device to produce spontaneous electrical oscillations,resonant means connected in said circuit to determine the frequency ofsaid oscillations, an independent source of periodic energy in circuitwith the other of said two control electrodes, the potential of saidelectrodes being such as to cause substantial distortion in the currentof the circuit interconnecting said cathode and anode.

6. In combination with an electrical discharge device having a cathode,an anode and at least two control electrodes, a source of energy and aconnecting circuit between said anode and cathode, a coupling betweensaid circuit and one of said control electrodes adapted to cause saiddischarge device to produce spontaneous electrical oscillations,resonant means connected in said circuit to determine the frequency ofsaid oscillations, a source of periodic energy in circuit with the otherof said two control electrodes, the potential of said electrodes beingsuch as to cause substantial distortion in the current of the circuitinterconnecting said cathode and anode, and the tuned frequency of saidoscillations being substantially a multiple of the frequency of saidlast-named source.

RALPH N. HARMON.

